Heretofore, because of having a high power generation efficiency and emitting no hazardous substances, a fuel cell has been put to practical use as a power generating device for industrial and household use, or as a source of power for an artificial satellite or a spacecraft, and in recent years, the development of fuel cells is being pursued as a source of power for a vehicle such as a passenger vehicle, a bus, a truck, a riding cart, or a luggage cart. Besides, although the fuel cell may be embodied as an alkaline fuel cell (AFC), a phosphoric acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), or a direct methanol fuel cell (DMFC), the commonly used fuel cell is a polymer electrolyte membrane fuel cell (PEMFC).
In this case, a solid polymer electrolyte membrane is interposed between two gas diffusion electrodes to be bonded to form a single unit. Then, when hydrogen gas serving as a fuel is supplied onto the surface of one of the gas diffusion electrodes serving as a fuel electrode (anode), the hydrogen is dissociated into hydrogen ions (protons) and electrons, and the hydrogen ions permeate through the solid polymer electrolyte membrane. In addition, when air serving as an oxidizing agent is supplied onto the surface of the other of the gas diffusion electrodes serving as an oxygen electrode (cathode), oxygen in the air is combined with the hydrogen ions and electrons to form water. Thus, an electromotive force is generated by such an electrochemical reaction.
Besides, because both sides of the solid polymer electrolyte membrane need to be maintained in wet condition in the case of the polymer electrolyte membrane fuel cell, water is supplied onto each of the fuel electrode side and the oxygen electrode side. In this case, the water moves as proton-carrying water from the fuel electrode side to the oxygen electrode side, and moves as back-diffusing water from the oxygen electrode side to the fuel electrode side.
It is known that when the amount of the back-diffusing water is increased, a flow path for the hydrogen gas is locally blocked by water on the fuel electrode side, resulting in a reduction in fuel cell performance or degradation of the fuel electrode. As a result, technologies have been proposed in which a conductive material formed with a mesh is arranged in the hydrogen gas flow path between a separator and the fuel electrode so that the water is appropriately diffused (refer, for example, to Patent Document 1).
[Patent Document 1]
Japanese Patent Application Publication No. JP-A-2005-209470